PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 169 



mesophytes, the plants of medium conditions. It is evi- 

 dent that mesophytes gradually pass into hydrophytes on 

 the one side, and into xerophy tes on the other ; but it is 

 also evident that mesophyte associations have the greatest 

 range of water supply, extending from a large amount of 

 water to a very small amount. 



It should be understood that these three groups of 

 associations, which are distinguished from one another by 

 the amount of the water supply, are artificial groups rather 

 than natural ones, for they bring together unrelated asso- 

 ciations, and often separate those that are closely related. 

 For example, a swampy meadow is put among hydrophyte 

 associations by this classification ; and it may shade into an 

 ordinary meadow, which belongs among the mesophytes. 

 Probably the largest fact that may be used in grouping 

 plant associations is that certain associations are so situ- 

 ated that they seek for the most part to reduce transpira- 

 tion, and that others are so situated that they seek for the 

 most part to increase transpiration. 



However, the factors that determine associations are 

 so numerous that they cannot be presented in an elementary 

 book, and the simpler artificial grouping given above will 

 serve to introduce the associations to observation. 



